The effects of CNS treatment (cranial irradiation and intrathecal chemotherapy) on the cognitive and perceptual functioning of children with leukemia are investigated. One study compares a group of leukemic children in remission with a matched group of sibs on measures from a battery of psychological tests (Wechsler, Bender-Gestalt and Graham-Kendall). These groups are followed-up 12-15 months after the initial assessment. A sample of Ewing sarcoma patients and their sibs and a sample of cystic fibrosis patients and their sibs are also being assessed in these procedures to control for effects other than CNA treatment. Computerized tomographic scans on the leukemia patients as a measure of change in brain structure will also be related to their intellective functioning. A second study deals with another sample studied prospectively. In this instance children in remission from leukemia are evaluated on the same battery of psychological tests as in the first study. They are tested just prior to prophylactic CNS treatment and are re-evaluated one year later to determine if any possible brain changes brought on by the CNS treatment have resulted in learning disabilities and diminished intellectual functioning. A normal control group will be tested at both points in time in order to check for testing effects.